Expand/consolidate "detecting bootc"

Continually comes up.

Signed-off-by: Colin Walters <[email protected]>

Author: cgwalters

docs/src/filesystem-sysroot.md

@@ -75,10 +75,4 @@ is recommended, along with an `ExecStartPre=mount -o remount,rw /sysroot`.
 
 ### Detecting bootc/ostree systems
 
-For tools like `cloud-init` that want to operate generically,
-conditionally detecting this situation can be done via e.g.:
-
-- Checking for `/` being an `overlay` mount point
-- Checking for `/sysroot/ostree`
-
-
+See the [package managers](package-managers.md) section on "Detecting image based systems".

docs/src/package-managers.md

@@ -26,6 +26,17 @@ that make the system appear writable, but it's arguably clearer not to do so by
 default. Detecting `/usr` as read-only here and providing the same information
 would make sense.
 
+To specifically detect if bootc is in use, you can parse its JSON status
+(if the binary is present) to tell if a system is tracking an image.
+The following command succeeds if an image is *not* being tracked:
+`test $(bootc status --format=json | jq .spec.image) = null`.
+
+### The `/run/ostree-booted` file
+
+This is created by ostree, and hence created by bootc (with the ostree)
+backend. You can use it to detect ostree. However, *most* cases
+should instead detect via one of the recommendations above.
+
 ### Running a read-only system via podman/docker
 
 The historical default for docker (inherited into podman) is that
@@ -72,15 +83,6 @@ $ podman run --read-only --rm --tmpfs /var -ti debian /bin/sh -c 'apt update &&
 error: read-only /usr detected, refusing to operate. See `man apt-image-based` for more information.
 ```
 
-### Detecting bootc specifically
-
-You may also reasonably want to detect that the operating system is specifically
-using `bootc`. This can be done via e.g.:
-
-`bootc status --format=json  | jq -r .spec.image`
-
-If the output of that field is non-`null`, then the system is a bootc system
-tracking the specified image.
 
 ## Transient overlays